Previous work by CGD faculty has established two critical properties of quantitative trait loci (QTL).[unreadable] First, they represent only a sub-set of the totality of genes functionally required for a complex trait such as the[unreadable] regulation of blood pressure or relative susceptibility to atherosclerosis. And second, the same sub-set of[unreadable] genes determine QTLs across mammalian species. QTLs for a given trait are often located at homologous[unreadable] chromosomal locations in human, mouse and rat. A probable explanation for these findings is that QTLs[unreadable] represent key regulatory functions whose role has been conserved over the course of mammalian evolution.[unreadable] Given that the QTLs for various phenotypes are responsible for most of the population variation on [unreadable] which evolution can act and that the identity of these QTL is an ancient property of mammals, genome[unreadable] dynamics predicts that the QTLs for fundamental physiological properties are likely to be genetically linked to[unreadable] promote the co-inheritance of favorable allelic combinations. The hypothesis of this project is that functional[unreadable] domains are a general feature of mammalian chromosomal organization and will be revealed by the clustering[unreadable] of QTLs affecting a phenotype. Rigorously testing this hypothesis requires that we efficiently locate QTLs at a[unreadable] resolution of a few Mb or less across the diversity of Mus subspecies.